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This question arises in almost every build log on the forum and every time someone explains why they are unsuitable for most CNC Routers. Here are some simulations I have run to prove the point...

All rails are 20mm diameter with 150mm between bearing block outer faces. The bearings are attached to a 75x160x20mm plate to which the forces are applied.

Assumptions:
Rail / box section ends are perfectly rigidly supported - in reality wont be so deflection will be greater than calculated.
All joints are perfect, modelled as single entity - in reality will get some deflection between joints.

The following diagrams each have different scales to make the deflection visible throughout.

Unsupported rail 200N in Z direction to simulate weight of Z-axis:

Supported rail 200N in Z direction to simulate weight of Z-axis:

Unsupported rail 200N in Z direction to simulate weight of Z-axisand 50N in parallel to X for cutting force:

Supported rail 200N in Z direction to simulate weight of Z-axis and 50N in parallel to X for cutting force:

So by the time you have factored in unsupported rails, rolled ball screws, or Acme screws, off the shelf commercial extrusion, lovejaw couplings, roller skate bearings as thrusts for ballscrews you have the equivalent of a high tensile wet rice krispies box.

Good job the builders of these machines don't have access to a FEMA program so they can see where they have gone wrong. :rofl:

Very pretty but also a very non accurate picture has been painted here. I use 30mm un supported rails on my machine and the accuracy is as good as can be (way more than my needs). The main factor is that it is all bolted to a very sturdy structure. the problem with your assumption here is that the rails sit on something with a solid base. The main problem with machines is the deflection of the bed and not necessarily the rails? if you look at all those nice machines with expensive supported rails or slides, most of them are mounted on aluminium profile that will flex more than the rail. You need to recalculate this to show the defection of the bed for it to stand up? Also what is this for Milling or routing?

Very pretty but also a very non accurate picture has been painted here...

You seem to be under the impression that I'm trying to say that unsupported rails are weak. I'm not, I'm saying they are many times weaker than their supported equivalent. If 30mm unsupported rails give you enough accuracy, then I'm saying you could achieve the same result on a much smaller diameter with support.

If I'm honest, I've seen so much abysmal design in commercial machines that I simply ignore what 'expensive' machines do. Their objective is to make money not make good machines, and those different objectives result in different designs.

Yes, there is deflection in the bed, I'm not implying this accounts for all the deflection. My only point here is that the same diameter of rail is tens of times weaker when unsupported, so all other factors being equal, if given a choice between unsupported and supported, you should go for supported.

An FEA simulation is never 100% accurate, but it's certainly a big improvement on estimation and common sense. I've simulated bed designs too, here's park of one I'm working on:

Jonathan i make no accusations, i just responded to your weak bold statement that lacked accountability of all facts. It was you that made the statement that unsupported rails are weak not me. I just pointed out some of the missing facts? If you could show how strong supported rails are attached to 40 x 40 profile like a lot of machines are you will see just how weak they are in comparison to your model?

PS I'm a performance engineer by trade and do a lot of modeling. i would get shot down in flames if i presented statements like that without all the facts.